1. Technical Field of the Invention
The present invention relates generally to a quickly activatable structure of a gas sensor element which may be built in a gas sensor employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O2, NOx, or CO, and a production and a reconditioning method thereof.
2. Background Art
In order to operate a three-way catalyst effectively for converting air pollutants contained in exhaust emissions of automotive engines into harmless products, it is essential to control burning of the engine so as to keep an air-fuel ratio of a mixture within a combustion chamber of the engine within a limited range.
Such air-fuel ratio control typically employs a gas sensor designed to measure the concentration of oxygen (O2) or an unburned gas contained in exhaust emissions of the engine. A gas sensor of such a type is equipped with a gas sensor element having an electrochemical cell made up of a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body. The electrochemical cell works as an oxygen sensor cell which measures the concentration of O2 or an unburned gas within the exhaust emissions. The air-fuel ratio is determined using an output of the gas sensor and used in controlling the burning of the engine.
For example, U.S. Pat. No. 6,332,965 B1 discloses a gas sensor element of the above described type.
Usually, it is impossible to determine the air-fuel ratio until the temperature of the gas sensor element reaches an activation temperature thereof. It is, therefore, difficult to perform the air-fuel ratio control immediately after startup of the engine. In most cases, the air-fuel ratio control is initiated after the temperature of the gas sensor element reaches the activation temperature.
Specifically, conventional air-fuel ratio control systems have difficulty in controlling the air-fuel ratio correctly until the temperature of the gas sensor element reaches the activation temperature thereof after startup of the engine, which may result in a difficulty in operating the three-way catalyst effectively, so that exhaust gasses containing a high concentration of pollutant are discharged directly to the air.
In recent years, the air-fuel ratio control is sought to be initiated immediately after startup of the engine in order to reduce the concentration of pollutant contained in exhaust gasses of the engine greatly. This requires quick activation of the gas sensor element for measuring the concentration of oxygen correctly immediately after startup of the engine.
In a case where the gas sensor element is used in an exhaust pipe of automotive engines, the gas sensor element usually undergoes a thermal load cyclically over a wide temperature range from room temperature to temperature of exhaust gasses. This results in an increase in electrical resistance of the gas sensor element, thus causing the activation temperature of the gas sensor element to be increased undesirably, which leads to a difficulty in activating the gas sensor element quickly immediately after startup of the engine.